6. A bond comprising a high-temperature metallic material and a ceramic,
hermetically joined with a glass ceramic using a glass composition
according to claim 1, said composition devitrifying during the sealing
operation performed at high temperatures.

7. The bond according to claim 6, wherein a metal and a ceramic are
joined together.

8. The bond according to claim 7, wherein a high-temperature nickel-based
metallic material and an oxide ceramic are joined together.

9. The bond according to claim 8, wherein the oxide ceramic has a
perovskite-like structure or a brownmillerite structure or an Aurivillius
structure.

10. The bond according to claim 8, wherein the ceramic has a stabilised
cubic or tetragonal zirconium oxide structure.

11. A bond comprising at least two ceramic/metal composite materials,
hermetically joined with a glass ceramic using a glass composition
according to claim 1, said composition devitrifying during the sealing
operation performed at high temperatures.

12. The bond according to claim 11, wherein a metal and a ceramic are
joined together.

13. The bond according to claim 12, wherein a high-temperature
nickel-based metallic material and an oxide ceramic are joined together.

14. The bond according to claim 13, wherein the oxide ceramic has a
perovskite-like structure or a brownmillerite structure or an Aurivillius
structure.

15. The bond according to claim 13, wherein the ceramic has a stabilised
cubic or tetragonal zirconium oxide structure.

Description:

[0001] The invention relates to a high-temperature-resistant devitrifying
solder glass that has a specific composition according to claim 1 and can
be used as a sealing solder glass.

[0002] It involves using a glass that devitrifies during the sealing
operation performed at high temperatures, causing crystal phases with
high coefficients of thermal expansion to precipitate.

[0003] Solder glasses and devitrifying solder glasses are now often used
to produce bonds where, for example, two metals or alloys of differing
composition or two ceramics of differing composition or structure or else
a metal and a ceramic are joined together. One or both of the materials
to be joined may also consist of a metal/ceramic composite.

[0004] Oxygen-transporting ceramic membranes are used in particular in
high-temperature processes. They represent, for instance, a
cost-effective alternative to cryogenic air separation for the recovery
of oxygen and are used in the production of syngas by partial oxidation
of hydrocarbons, such as methane, according to the following reaction:

2CH4+O2→2CO+4H2 (1)

[0005] Other applications are, for example, the recovery of oxygenated air
as described, for instance, in DE 102005 006 571 A1, the oxidative
dehydrogenation of hydrocarbons or hydrocarbon derivatives, the oxidative
coupling of methane to C2+ and the decomposition of water and
nitrous oxide.

[0006] Ceramic membranes are often used as tubes, these often being
integrated into modules. Ceramic hollow fibres with a diameter of less
than 5 mm represent a special form of tube. Such modules should be
chemically and thermally resistant while at the same time guaranteeing a
hermetic seal. Tube or hollow-fibre membranes can be integrated into
modules by embedding--or potting--them in a casting compound, also known
as a potting compound or bonding material.

[0007] Ceramic materials which are the same as or similar to the ceramic
membrane material itself are considered to be suitable materials for this
purpose as they exhibit optimum compatibility. However, there is a
problem in that such layers cannot be hermetically sinter-sealed without
irreversibly changing the ceramic hollow-fibre membranes themselves. A
method for creating such modules using ceramic material as a potting
compound is described, for example, in EP 0941759 A1.

[0008] WO 2006089616 describes a potting that consists of at least three
layers containing at least two different casting compounds. The two outer
layers can be formed from ceramic material and the layer in the middle
can be formed from glass. A drawback of this method of potting is that on
account of its oxides, such as zirconium oxide or iron oxide, glass
represents an extremely reactive component and destroys the oxidative
constituents of the ceramic material.

[0009] Therefore, the design of chemically and thermally resistant modules
with ceramic tube, hollow-fibre or capillary membranes requires an
adaptation of the potting materials.

[0010] Normally, glasses that melt at a lower temperature have higher
coefficients of thermal expansion than glasses that melt at a higher
temperature. Consequently, when a solder glass is to be employed as the
sealing joint for a material bond at a higher temperature (e.g.
800° C.), there are no glasses that have, for example, a melting
temperature >800° C. and at the same time a coefficient of
thermal expansion >10×106 K-1. In such cases, a
mechanically and thermally stable sealing joint cannot be produced via a
solder glass but it can via a devitrifying solder glass.

[0011] In order to produce a devitrifying solder glass, a glass of a
suitable composition is first melted and then cooled to room temperature
without it devitrifying before being pulverised with the aim of achieving
typical particle sizes of between 1 and 200 μm. The glass powder is
then applied to one or both of the workpieces to be joined. A number of
additives, such as aqueous or non-aqueous solvents, oils or polymer
solutions, can be used for this. However, it is also possible to apply
ceramic films to one or both of the workpieces to be joined.

[0012] In a further step the workpieces to be joined are then heated with
the solder glass to a suitable temperature. The glass particles thus
sinter together and bond with the two workpieces to be joined. However,
it is also possible not to put the workpieces together until a high
temperature has been reached. The sintering should occur through the
viscous coalescence of the glass. Once the glass particles have largely
sintered together and bonded with the workpieces to be joined,
devitrification should occur. The devitrification process can, however,
also be induced through a temperature change, with a temperature above or
below the actual joining temperature being used depending on the chemical
composition of the solder glass. On completion of the joining process,
the workpieces are joined tightly together.

[0013] Glass ceramic materials with widely varying compositions count as
state of the art. For example, glass ceramics from the
BaO--CaO--Al2O3--SiO2 system are used to join
high-temperature fuel cell stacks. In addition to a high temperature
resistance, this material needs to meet the following demands. The
joining material needs to be extremely stable; it should have an
electrically isolating property and it must not react with gases, such as
H2, O2, H2O and CH4. In addition, it should bond well
with the metallic surface of the fuel cell stack (Schwickert T. et al.
Mat.-wiss. u. Werkstofftech. 33, 363-366, 2002).

[0014] A glass ceramic that is specifically suitable for use in
embedding--or potting--ceramic membranes in solid metallic forms again
needs to meet special requirements. Alongside a temperature resistance of
up to 900° C. and a hermetic seal, the glass ceramics used must be
chemically inert to oxide ceramics that have a perovskite structure, a
brownmillerite structure or an Aurivillius structure, and/or also be
chemically inert to high-temperature metallic materials. This counteracts
the problem of material destruction mentioned above.

[0015] Moreover, the glass ceramics must have a coefficient of thermal
expansion that is equivalent or similar to that of oxide ceramics and/or
a coefficient of thermal expansion that is equivalent or similar to that
of high-temperature metallic materials.

[0016] Metals mostly have linear coefficients of thermal expansion of
between 10×10-6 and 16×10-6 K-1. If the
coefficients of expansion do not match that of the solder material,
stress will occur on temperature changes and this will ultimately lead to
the destruction of the bond. In general, differences in the linear
coefficient of thermal expansion of less than 1-2×10-6K-1
can be tolerated. If the workpieces to be joined have different
coefficients of thermal expansion, the expansion coefficient of the
devitrified solder glass should preferably be in the middle.

[0017] The sintering and devitrification of the solder glass are not
always separate or separable processes with respect to time and
temperature. Rather, they usually take place simultaneously, the
sintering rate increasing alongside the temperature. The same also
applies to the speed of devitrification of the glass. Therefore, a time
and temperature frame in which the sintering process takes place
considerably faster than devitrification should be found in the case of
each concrete joining problem. A devitrifying sealing solder glass must
therefore have the right (high) expansion coefficient, be able to be
sintered under the respective applicable conditions before
devitrification occurs and also be sufficiently thermally stable, i.e.
not melt, at use temperature.

[0018] Oxidic crystal phases that have a high thermal expansion and can be
precipitated from oxidic glasses are primarily earth alkali silicates.
One finds in the literature quantitative descriptions of the phases
BaSi2O5 and Ba3Si5O.sub.13 in G. Oelschlegel,
Glastechnische Berichte 44 (1971), 194-201, as well as
Ba2Si3O8 in G. Oelschlegel, Glastechnische Berichte 47
(1974), 24-41, also with regard to their linear coefficients of thermal
expansion. One also finds in the literature descriptions of glass
ceramics with other earth alkali oxides (SrO, CaO) that also have
coefficients of thermal expansion >10×10-6, for example in
Lahl, J. Mater. Sci. 35 (2000) 3089, 3096. In addition to the desired
crystal phase and high coefficients of thermal expansion, these glass
ceramics also consist of other phases. These may be crystal phases of
other compositions or glass phases, and in most cases they have much
lower coefficients of thermal expansion. The reason for this consists in
the fact that a glass of, for example, the composition 50 BaO×50
SiO2 devitrifies much too quickly to sinter hermetically as powder.
The devitrification process would, in this case, begin much too soon and
prevent sintering.

[0019] The devitrification process can be slowed down by relatively small
amounts of additives, such as boric oxide or aluminium oxide. This is,
however, also associated with a reduction in the coefficient of thermal
expansion.

[0020] It is also known that these components, if anything, aid
devitrification in other glass compositions. For example, one very often
finds in the literature that ZrO2 acts as a nucleant, Maier, cfi
Ber. DKG 65 (1988) 208, Zdaniewski, J. Am. Ceram. Soc. 58 (1975) 16,
Zdaniewsi, J. Mater. Sci, 8 (1973) 192. In the
MgO/Al2O3/SiO2 system volume nucleation cannot even be
induced without adding ZrO2 Amista et al. J. Non-Cryst. Solids
192/193 (1995) 529. Here, surface devitrification is observed in the
absence of ZrO2 (or TiO2). The volume nucleation rate is in
this case increased by many orders of magnitude by adding a few %
ZrO2.

[0021] The present invention has the objective of developing a
devitrifying solder glass that exhibits all of the above properties and
avoids the above problems associated with current state-of-the-art glass
ceramics.

[0023] In accordance with the invention the additives known in the art can
be combined with other additives, primarily La2O3 and/or
ZrO2. Surprisingly, even small additions of ZrO2,
La2O3 or rare earths are extremely effective. However, the
additives La2O3 or ZrO2 also suppress devitrification
without the simultaneous presence of B2O3 or Al2O3,
and thus permit the use of a devitrifying solder glass.

[0030] It is an advantage to produce the devitrifying solder glasses from
melted, pulverised glass with a particle size of 1 and 200 μm,
preferably these are produced from melted, pulverised glass with a
particle size of 10 and 150 μm and especially favoured is melted,
pulverised glass with a particle size of 30 and 125 μm--the rule being
the finer the particle size, the quicker the devitrification.

[0031] The high-temperature-resistant devitrifying solder glass is
advantageously used as a hermetic sealing solder glass to join
high-temperature metallic materials and ceramics or else ceramic/metal
composite materials. Preferably, a metal and a ceramic are joined
together during this process. Especially favoured are a high-temperature
nickel-based metallic material and an oxide ceramic, the oxide ceramic
advantageously having a perovskite-like structure or a brownmillerite
structure or else an Aurivillius structure and the ceramic preferably
having a stabilised cubic or tetragonal zirconium oxide structure.

[0032] The present invention is to be described below using the following
examples of embodiments.

Embodiment Example 1

[0033] A ceramic hollow fibre suitable for separating air in the pressure
gradient (mixed electron/oxygen ion conductors) is to be joined to a
high-temperature nickel/iron-based alloy. Both of the materials to be
joined have linear coefficients of thermal expansion of
14-15×10-6K-1 in the temperature range of 25 to
850° C.

A 2 mm thick hole is drilled through the metal. In the same place the
metal is drilled approximately 4 mm deep using a drill with a diameter of
8 mm in order to produce a conical cavity, at the cone point of which the
2 mm drill hole is located. Now, a ceramic hollow fibre with a diameter
of 1.8 mm is inserted into this drill hole. 0.3 g of a glass powder
composed of
15ZnO.25BaO.1B2O3.1ZrO2.1La2O3.57SiO2 is
put into the conical cavity. For this, a grain size fraction of 50-80
μm obtained through screening is used. Then the assembly of metal,
hollow fibre and glass is put in an oven and heated to a temperature of
900° C. The heating rate is 5K/min. The end temperature is
maintained for 1 h and the oven is then cooled. A hermetic sealing joint
is obtained. The bond can be used at temperatures of up to 900° C.

Embodiment Example 2

[0034] A ceramic hollow fibre and a high-temperature alloy with properties
as described in embodiment example 1 are to be joined together. A
cylindrical hole with a depth of 4 mm and a diameter of 10 mm is drilled
in the metal. Then, in total seven holes, each with a diameter of 1.5 mm,
are drilled in the bottom of this drill hole. Hollow-fibre membranes with
a diameter of 1.3 mm are inserted through these holes. A glass composed
of 36.25.BaO.7.5
Al2O3.5B2O3.2ZrO2.2La2O3.3BaF2.44-
.25SiO2 with a grain size fraction of 30-125 μm is used to produce
the sealing joint. From this, a pourable slurry is produced using a 2%
solution of polyvinyl alcohol in water and this is filled into the
cylindrical hole. After drying, the assembly is brought to a temperature
of 950° C., the rate of heating being 1K/min up to 600° C.
and 5K/min at a higher temperature.

Embodiment Example 3

[0035] A ceramic hollow fibre and a high-temperature alloy with properties
as described in embodiment example 1 are to be joined together. A
hollow-fibre bundle is inserted into a polymer mould (O=25mm). A ceramic,
non-aqueous slurry based on ethanol, polyvinyl butyral and hydroxypropyl
cellulose is produced from a glass composed of
41.75.BaO7.5Al2O35B2O31ZrO21La2O3.42.2-
5SiO2 using a grain size fraction of 30-50 μm, which was produced
through screening. The slurry is poured into the polymer mould. It is
then dried and the solid form is taken out of the mould and sintered in
the oven at 920° C. After sintering, the solid form has a diameter
of 22 mm. The solid sintered form is then put on a metal plate with a
hole (O=16 mm) so that the hollow fibres, the inner edge of the metal
plate and the glassy crystalline solid form (O=22 mm) overlap by
approximately 3 mm. In a second temperature treatment step this assembly
is then heated to 980° C. and left at this temperature for 1 h.

Embodiment Example 4

[0036] A flat ceramic membrane (thickness 1 mm) produced by means of film
technology is to be joined to a high-temperature alloy. Both materials
have linear coefficients of thermal expansion of
14-15×10-6K-1 in the temperature range of 25 to
850° C. For this, a pourable slurry based on ethanol/propanol with
the addition of hydroxypropyl cellulose, polyvinyl alcohol, octyl
phthalate, tensides and polyethylene glycol is produced from a glass
composed of
19ZnO.25BaO.1B2O3.2ZrO2.2La2O3.51SiO2. This
is used to produce a ceramic film using the doctor blade process.
Contours are cut out of this film using a CO2 laser. These films are
then put on the metal plate and the flat ceramic membranes are
subsequently applied.

[0037] This assembly is sintered at 950° C. and kept at this
temperature for 1 h. The rate of heating amounted to 1K/min up to a
temperature of 650° C. and 5K/min thereafter.

Embodiment Example 5

[0038] A high-temperature alloy (linear coefficient of thermal expansion:
11.5×10-6 K-1) is to be joined to a flat membrane made of
stabilised tetragonal zirconium oxide ceramic (thickness 200 μm,
linear coefficient of thermal expansion: 10×10-6K-1)
produced by means of film technology. For this, a paste based on
ethanol/propanol with the addition of hydroxypropyl cellulose, polyvinyl
alcohol and octyl phthalate is produced from a glass composed of
35BaO.3B2O3.2ZrO2.2La2O3.7Al2O3.51SiO.-
sub.2. This paste contains 50 vol % glass and is used to produce a sealing
joint between the zirconium oxide ceramic and the high-temperature alloy.
This assembly is sintered at 950° C., kept at this temperature for
1 h, then brought to a temperature of 880° C. and kept at this
temperature for a further 5 h. The rate of heating in each case amounted
to 2K/min.

Patent applications by Bernd Langanke, Holzwickede DE

Patent applications by Steffen Schirrmeister, Muelheim An Der Ruhr DE

Patent applications by Thomas Schiestel, Stuttgart DE

Patent applications by BORSIG Process Heat Exchanger GmbH

Patent applications by THYSSENKRUPP UHDE GMBH

Patent applications in class Next to another silicon containing layer

Patent applications in all subclasses Next to another silicon containing layer